The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Computer hard disk drives (HDDs) employ read/write heads to read and write data on magnetic layers of one or more rotating disks. The data is stored on concentric data tracks. The read/write heads are positioned at centers of the data tracks during read and write operations. The HDDs include closed-loop servo systems that position the read/write heads on the data tracks using servo information that is embedded in a dedicated portion of each data track.
Close proximity of a read/write head to a disk enables high-resolution servo patterns and user data to be recorded. The servo patterns are written in servo sectors (wedges), which are interleaved between data sectors. The servo patterns provide a servo controller with position information that enables a head positioner, such as a rotary voice coil motor, to move a read/write head from track-to-track during random access seek operations. The servo patterns also maintain the read/write head in proper alignment with a track during track following operations, when user data is read from or written to available data sectors. The servo patterns allow a read/write head to follow a centerline of both circular and non-circular tracks. For example, a track may be non-circular due to spindle wobble, disk slip and/or thermal expansion.
The servo information is written on a surface of a disk as a set of wedges (tracks) that extend radially from an inner diameter of the disk. A portion of each wedge has an automatic gain control (AGC) field, a synchronization field, an index mark, a gray-coded track number, and a set of fine-positioned offset bursts. The offset bursts are configured in an echelon across a data track. Read/write head position is adjusted relative to the center of the data track based on respective amplitudes and associated times of the offset bursts.
Traditionally, a machine referred to as a “servo writer” is used to write the servo information to a disk. The servo writer may include the following features: a massive granite base to minimize effects of vibration; precision fixtures to hold a HDD; a precision laser interferometer-based actuator mechanism to place a read/write head radially with respect to an axis of rotation of the disk; and an external clock head to position servo wedges in time. The servo writers tend to be large, expensive, and require a clean room environment. As density of the tracks on the disk increases, the time required by the servo writer to write the servo data to the disk also increases, which can create a bottleneck.
To reduce manufacturing time, a technique referred to as self-servo writing (SSW) has been developed. Servo patterns are written by a HDD during SSW without use of a traditional servo track writer. Self-servo writing involves reading position and timing information from the disk, positioning a read/write head using the position information, and writing servo patterns to the disk using the timing information.
One self-servo writing technique includes writing spiral servo tracks on a disk. The spiral servo tracks are initially written via an external servo writer. Instead of slowly writing servo information to each concentric data track on each surface of each disk in a hard drive, a limited number of spiral servo tracks are written. The spiral servo tracks are associated with a single surface of one of the HDD disks.
Without the aid of a traditional servo writer, the HDD uses timing information in the spiral servo tracks to determine a radial and circumferential position of the read/write head. The read/write head is then positioned and writes conventional servo data (servo wedges) to concentric data tracks on the disks via the servo system. Since a smaller number of spiral servo tracks are initially written, servo-writing time is reduced. Once the self-servo writing is finished, the spiral servo tracks can be overwritten by data tracks.
Another SSW technique includes the writing of sectional spiral servo tracks. For example, a HDD may write short sectional spiral servo tracks followed by conventional servo wedges that are based on the sectional spiral servo tracks. However, this may create interruptions in writing of a concentric servo pattern by design. Interruptions can also occur by accident, such as, for example, due to a power failure or a system reboot. The interruptions can cause gaps or overlapping of servo wedges. Misalignment can occur between tracks written prior to an interrupt and servo wedges written after the interrupt. For example, a new track written after an interrupt may not be properly aligned in radial and circumferential directions relative to a servo wedge written before the interrupt. Track misalignment can result in “track squeeze”, which refers to differences in relative distances between servo wedges. Track squeeze can result in synchronization, track following, and track reading and writing problems.
Internal HDD servo wedge writing also suffers from self-propagation. During self-propagation, servo bursts in a previous servo wedge are used to position a read/write head as servo bursts are written to the next servo wedge. However, perturbations in the servo bursts in the previous servo wedge propagate to the servo bursts in the next servo wedge. Compound errors that propagate across the servo wedge can lead to excessive wedge non-circularity.